Man-made structures such as boat hulls, buoys, drilling platforms, oil production rigs, and pipes which are immersed in water are prone to fouling by aquatic organisms such as green and brown algae, barnacles, mussels, and the like.
Such structures are commonly of metal, but may also comprise other structural materials such as glass reinforced plastic (GRP), concrete or wood. This fouling is a nuisance on boat hulls because it increases frictional resistance during movement through the water, the consequence being reduced speeds and/or increased fuel costs. It is a nuisance on static structures such as the legs of drilling platforms and oil production rigs, firstly because the resistance of thick layers of fouling to waves and currents can cause unpredictable and potentially dangerous stresses in the structure, and, secondly, because fouling makes it difficult to inspect the structure for defects such as stress cracking and corrosion. It is a nuisance in pipes such as cooling water intakes and outlets, because the effective cross-sectional area is reduced by fouling, with the consequence that flow rates are reduced.
It is known to use antifouling coatings, for instance as a top coat on underwater hulls, to inhibit the settlement and growth of marine organisms such as barnacles and algae, generally by release of a biocide for the marine organisms. Many successful antifouling coatings in recent years have been “self-polishing copolymer” paints based on a polymeric binder to which leaving group moieties are chemically bound, and from which leaving group bonds to the polymer backbone are gradually hydrolysed by seawater. In such binder systems, the side groups of a linear polymer unit are split off in a first step by reaction with seawater, the polymer framework that remains becoming water-soluble or water-dispersible as a result. In a second step, the water-soluble or water-dispersible framework at the surface of the coating layer on the ship is washed out or eroded. Such paint systems are described for example in GB-A-1 457 590, EP779304, WO2005005516, WO200202698, WO2004018533 or WO201018144 and WO9937723.
Current marine solvent-borne antifouling coatings release significant volumes of organic solvent into the atmosphere upon coating application. This is harmful to the environment and will be legislated against increasingly in the coming years. The volatile organic content (VOC) of antifouling coatings is already restricted by legislation in many countries. For example, the European Solvents Emissions Directive limits the VOC to less than 270 g/kg—equivalent to about 450 g/l—for the overall emissions from shipyards which apply antifouling coatings compositions, whilst in the South Coast Air Quality Management District (SCAQMD) of California, USA an upper limit of 330 g/l for pleasure craft antifouling paints is in force.
Waterborne formulations offer the potential to significantly reduce the organic solvent content of antifouling coatings. Such coatings can also be referred to as water-based or aqueous coatings. The waterborne antifouling coating compositions, by definition comprise water (usually more than 20 weight % based on the weight of all the components in the coating composition). The waterborne coating compositions disclosed in this patent application typically have VOC<100 g/l, and also surprisingly <50 g/l, and more surprisingly <10 g/l (as formulated).
Besides these VOC demands, antifouling coatings on boats should provide adequate antifouling protection. Further, the coating should show hardly any so-called cold flow or plastic deformation, in other words, the film should not ripple when the ship moves in the water. Additionally, the coating composition needs to show a sufficiently short drying time. Further, the antifouling coatings should be stable during storage.
JP2009173914 and WO2006/077738 and EP1958991 disclose waterborne antifouling systems prepared by latex polymerisation.
Waterborne antifouling systems are currently believed to have inferior mechanical strength and film properties compared to solvent-borne systems. It is believed that the water sensitive nature of these systems means that water can be absorbed too easily and result in weakness of the paint films. It is known that the Pigment Volume Concentration (PVC) is key to the aesthetics and physical properties of a coating composition. PVC is defined as the ratio of pigment volume to the total dry film volume. The higher the PVC, the more the potential voids in the dried coatings and the higher the water permeability. Typical antifouling compositions have a PVC of no more than 60%.
WO 97/00919 discloses an antifouling paint having little or no biocidal activity, an essentially insoluble film-forming binder, the PVC being in the region of the critical pigment concentration (CPVC). However, WO 97/00919 fails to disclose waterborne coating compositions having a Pigment Volume Concentration greater than 80% as required by the present invention. All the coating compositions in the examples have a PVC ranging from between 54.6% to 78.2%. Further, all of the coating compositions exemplified in WO 97/00919 are solvent-based systems, and no waterborne systems are exemplified or enabled. In fact, the inventors have found that the solvent-based coating composition of Example 11 in WO 97/00919 (which has the closest PVC value to that required by the present invention), when applied and dried to a substrate, cracks and delaminates from the surface and therefore is unsuitable for use as an antifouling coating composition. It is therefore surprising that when the antifouling coating composition is not a solvent-based system, but a waterborne-system as defined according to the present invention, having a higher PVC than suggested in WO 97/00919, the coating produced therefrom has good mechanical strength and structural integrity as well as good antifouling protection.